This invention relates to a process for removing unwanted magnesium from an aluminum alloy without causing pollution of the air.
Magnesium, if contained in wrought products of aluminum alloy (containing pure aluminum), improves the mechanical properties of the material without impairing the corrosion-resistance thereof. In this respect, it is a useful alloying element. Aluminum alloys prepared so as to contain up to about 5% of magnesium find widespread acceptance as corrosion-resistant aluminum alloys.
In the case of aluminum alloys for use in castings, particularly in die castings, however, magnesium is an unwanted element. Inclusion of magnesium in these aluminum alloys, therefore, is rigidly controlled. For use in castings, aluminum alloys are prepared so as to contain a fairly large proportion of silicon with a view to improving their casting property. Magnesium, if present therein, reacts with silicon to produce an intermetallic compound Mg.sub.2 Si which embrittles the alloys. This explains why the presence of magnesium in such aluminum alloys is undesirable.
According to JIS (Japanese Industrial Standard) H-5302 titled "Die Casting Aluminum Alloys," the allowable magnesium content in ADC10 and ADC12 which together account for more than 95 percent of the die casting alloys actually used in Japan is specified as not to exceed 0.3 percent as shown in the table. Most standards adopted by ordinary consumers and those adopted by producers dictate still lower magnesium contents in aluminum alloys. In the case of 380-series alloys which are used in the largest quantities in the United States, the highest allowable magnesium content is 0.1 percent.
Table __________________________________________________________________________ Chemical analysis (%) Foreign Type Symbol Cu Si Mg Zn Fe Mn Ni Sn Al counerparts __________________________________________________________________________ Die casting aluminum 2.0 ASTM SC84A alloy, Type 10 ADC10 .about. 7.5.about. 9.5 0.3 1.0 1.3 0.5 0.5 0.3 Balance DIN GDA1Si6 Cu3 4.0 max max max max max max BNF A-S10U4-Y4 Federal A380 Die casting aluminum 1.5 10.5.about.12.0 ASTM SC114A alloy, Type 12 ADC12 .about. 0.3 1.0 1.3 0.5 0.5 0.3 Balance DIN - 3.5 (9.6.about.12.0) max max max max max max BNF A-S10U4-Y4 Federal __________________________________________________________________________ SC114A Former Balance of AA code No. Code No. Product Si Fe Cu Mn Mg Cr Ni Zn Sn Ti Others aluminum __________________________________________________________________________ 380.0n 380 n 7.5-9.5 2.0 3.0-4.0 0.50 0.10 -- 0.50 3.0 0.35 -- 0.50 Balance 380.2 380 Ingot 7.5-9.5 0.7-1.1 3.0-4.0 0.10 0.10 -- 0.10 0.10 0.10 -- 0.20 " A380.0n A380 D 7.5-9.5 1.3 3.0-4.0 0.50 0.10 -- 0.50 3.0 0.35 -- 0.50 " A380.1n A380 Ingot 7.5-9.5 1.0 3.0- 4.0 0.50 0.10 -- 0.50 2.9 0.35 -- 0.50 " __________________________________________________________________________
About 85 percent of all the die casting alloys are produced by using aluminum scraps as the raw material. In wrought products of aluminum alloys produced today, the average magnesium content is 0.96 percent. It follows as a consequence that the content of magnesium in alluminum scraps generally exceeds the allowable upper limit of magnesium content in die casting alloys. For use in the preparation of die casting alloys, therefore, such aluminum scraps must be given a treatment for removal of magnesium. The processes heretofore employed for the removal of magnesium are such that they have inevitably caused air pollution.
The removal of magnesium from aluminum alloys has heretofore been accomplished exclusively by either of two methods: (1) one method using chlorine gas and (2) another using a fluoride. The former method comprises blowing chlorine gas into a given molten aluminum alloy. Magnesium exhibits a stronger chemical affinity for chlorine than for aluminum. By making use of this difference of affinity, magnesium is removed in the form of MgCl.sub.2 from the aluminum alloy. The waste gas emanating from this treatment, consequently, contains unaltered free chlorine and aluminum chloride in large proportions and causes air pollution. When chlorine gas is blown into the molten aluminum alloy through a graphite pipe as is usually practiced, the chlorine concentration in the waste gas reaches the level of about 3000 ppm. To lower the chlorine concentration in the waste gas, there have been studied various measures, including those of Bell System Process, Alcoa Process, Derham Process, etc.
The Bell System makes use of a bell which is provided with an inlet pipe for chlorine gas and a discharge pipe for waste gas. The Bell System Process effects the removal of magnesium by placing molten aluminum inside the bell in a tightly sealed state and then introducing chlorine gas into the molten metal. This process has been adopted more widely than any other process in the United States.
The Alcoa Process involves an operation which comprises introducing molten aluminum into a closed reaction chamber divided into several sections and passing chlorine gas through the molten metal in the form of finely divided bubbles with the aid of a contactor kept in rotation. ((1) "Alcoa Fumeless Demagging Unit," Report No. 1, Alcoa Technology Marketing Division, Pittsburgh, Pa. 15129, June 1972; (2) Hatch J. E. "Chlorine Control by In-Line Fluxing Operations," Paper No. 73-325, presented at Air Pollution Control Association Seminar, Chicago, June 1973.)
The Derham Process comprises the steps of covering the surface of molten aluminum with a layer of molten flux about 20 mm in thickness and blowing chlorine gas into the molten aluminum for thereby removing magnesium from the molten metal. ((1) Derham L. J. and Derham M. G., "Purification of Aluminum" U.S. Pat. No. 3650730; (2) Andrews C. C., "The Derham Process" a bulletin, Aluminum Process Inc., Cleveland, Ohio, 44114, March 1973.)
With the demagging method which makes use of chlorine gas, however, the waste gas is certained to contain harmful free chlorine and aluminum chlorine.
In the method which resorts to use of a fluoride, aluminum fluoride is mainly used as the fluoride and the reaction shown below is utilized for the removal of magnesium. EQU 2AlF.sub.3 +3Mg=3MgF.sub.2 +2Al
This compound, aluminum fluoride, is so expensive as to render this method uneconomical. Moreover, aluminum fluoride undergoes decomposition to give rise to a fluorine compound which causes air pollution. For this reason, this method is not used so widely as the aforementioned method which effects the treatment by use of chlorine.
The primary object of this invention is to provide a process for the removal of magnesium from an aluminum alloy containing magnesium without any possibility of air pollution.
Another object of this invention is to provide a process for the removal of magnesium from a magnesium-containing aluminum alloy, which process can be carried out more readily with a simpler device than the conventional method.